1. Field of the Invention
The present invention relates to a laser power supply apparatus for supplying an AC voltage to a laser device for use in, for example, laser beam machining so as to cause the laser device to discharge and radiate laser light.
2. Description of the Prior Art
Referring now to FIG. 26, there is illustrated a diagram showing the structure of a laser device., As shown in the figure, the laser device is provided with a pair of electrodes 1, a pair of dielectric layers 2 on the pair of electrodes 1, respectively, a high-frequency AC power supply 3, and a partial reflection mirror 4 for causing the laser device to generate laser radiation 6 in cooperation with a total reflection mirror 5. FIG. 27 shows an equivalent circuit of the pair of dielectric layers 2 and a discharge area or space in which a discharge occurs to excite a gas contained in the discharge space to start laser action. In FIG. 27, reference numeral 7 denotes a dielectric capacitance caused by the pair of dielectric layers 2, and 8 denotes an equivalent resistance caused by the discharge area. Further, FIG. 28 is a schematic circuit diagram showing the detailed structure of the high-frequency power supply 3. As shown in the figure, the high-frequency power supply 3 is provided with a plurality of high-speed semiconductor switches 9-1 to 9-4, a DC power supply 10, a high-frequency transformer 11, and a pair of output reactors 12.
As shown in FIG. 28, the plurality of high-speed semiconductor switches 9-1 to 9-4 form a full-bridge inverter. The full-bridge inverter is driven by four control signals S1 to S4 as shown in FIG. 29, for example. The full-bridge inverter generates a rectangular high-frequency voltage Vout. The high-frequency transformer 11 increases and furnishes the rectangular high-frequency voltage Vout to the load by way of the pair of output reactors 12. The pair of output reactors 12 and the dielectric capacitor 7 eliminate high-frequency components in the output current flowing from the high-frequency transformer 11 to the load. An approximately sinusoidal current I.sub.d thus flows through the load and forms a discharge. The discharge generated excites a gas contained in the discharge space to start laser action. The combination of the partial reflection mirror 4 and the total reflection mirror 5 forces the excited gas to radiate in phase. The laser light 6 generated by the laser radiation can be used for laser beam machining, for example. In most cases, the leakage inductance of the high-frequency transformer 11 can be utilized as the pair of output reactors 12.
Although the required intensity and pulse width the laser beam 6 depends on objects to be machined, when a high degree of accuracy is required, the laser beam 6 should have a higher intensity and a shorter pulse width in most cases, as shown in FIG. 30. In order to increase the intensity of the laser light 6, a larger amount of current needs to be passed through the discharge space in the laser device.
Since the impedance of the discharge load is capacitive, when causing a larger amount of current to be passed through the discharge space, the higher the frequency of the AC voltage applied to the load, the lower the voltage Vc across the dielectric capacitor 7, as shown in FIGS. 31(a) to 31(d). As can be seen from FIGS. 31(a) to 31(d), the voltage Vc required to generate the same amount of current I.sub.d decreases with an increase in the frequency of the AC voltage applied to the load. When the thickness of each of the pair of dielectric layers 2 is restricted to a certain value, there is no other choice but to increase the frequency of the AC voltage in order to cause a larger amount of current to be passed through the load, because the voltage across each of the pair of dielectric layers 2 has to be less than its withstand voltage. On the other hand, when the frequency of the AC voltage applied to the load is increased, the amount of current flowing through the load is limited by the electrical size of the pair of output reactors 12, other than the discharge resistance 8. When the electrical size of the pair of output reactors 12 is relatively large, the voltage Vc across the dielectric capacitor 7 drops before the current Id rises up to an adequate value, and hence a large amount of current cannot be passed through the load. The leakage inductance of the high-frequency transformer 11 is utilized as the pair of output reactors 12 in most cases, as previously mentioned. Further, the leakage inductance of the high-frequency transformer 11 cannot be reduced to zero because of its structural limits. Thus the minimum of the total inductance of the pair of output reactors 12 is inevitably determined and the largest amount of current resulting from the minimum inductance is therefore limited. In general, since the leakage inductance of a transformer is proportional to the square of the winding ratio of the secondary winding to the primary winding, the electrical size of the pair of output reactors 12 is increased when the secondary winding is higher than the primary winding in voltage. As a result, the frequency of the AC voltage applied to the load has an upper limit and hence the intensity of the laser light 6 has an upper limit because of the withstand voltage of the dielectric capacitor 7. Accordingly, the prior art laser power supply apparatus using the high-frequency transformer 11 cannot cause gas laser devices to generate laser radiation having a sufficient intensity suitable for close tolerance machining.